Method and apparatus for selecting neighbor cells in mobile communication system

ABSTRACT

A neighbor cell search method and apparatus of a User Equipment (UE) is provided for performing the cell search procedure with the utilization of a Physical Cell Identity (PCI) list of PCIs used by evolved Node Bs (eNBs) for a Closed Subscriber Group (CSG) which is from an eNB in a Long Term Evolution (LTE) system. The cell search method includes selecting a cell of a base station, receiving system information including a Physical Cell Identity (PCI) list used by neighbor Closed Subscribed Group (CSG) cells of the base station, determining whether the terminal is in an any cell selection state for selecting any cell, and searching, when the terminal is in the any cell selection state, neighbor cells without application of the PCI list. The cell search method and apparatus are capable of performing the cell search procedure to appropriate eNBs, resulting in improvement of network attachment efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of a U.S.Provisional application filed on Oct. 5, 2011 in the U.S. Patent andTrademark Office and assigned Ser. No. 61/543,475, and a U.S.Provisional application filed on Oct. 10, 2011 in the U.S. Patent andTrademark Office and assigned Ser. No. 61/545,363, the entire disclosureof each of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication system. Moreparticularly, the present invention relates to a neighbor cell searchmethod and apparatus of a User Equipment (UE) with the utilization of aPhysical Cell Identity (PCI) list of PCIs used by evolved Node Bs (eNBs)for a Closed Subscriber Group (CSG) which is from an eNB in a Long TermEvolution (LTE) system.

2. Description of the Related Art

With the rapid advances of technologies, mobile communication systemshave evolved to the 4th Generation (4G) system represented by Long TermEvolution (LTE). LTE has been designed to work under the assumption ofvarious scenarios such as by taking notice of various base stationcapabilities. For example, LTE has introduced a concept of ClosedSubscriber Group (CSG) in which only terminals belonging to a CSG areentitled to access a corresponding CSG cell.

FIG. 1 is a diagram illustrating an architecture of an LTE systemaccording to the related art.

Referring to FIG. 1, the radio access network of the mobilecommunication system includes evolved Node Bs (eNBs) 105, 110, 115, and120, a Mobility Management Entity (MME) 125, and a Serving-Gateway(S-GW) 130. The User Equipment (hereinafter, referred to as UE) 135connects to an external network via eNBs 105, 110, 115, and 120 and theS-GW 130.

In FIG. 1, the eNBs 105, 110, 115, and 120 correspond to legacy node Bsof a Universal Mobile Communications System (UMTS). The eNBs 105, 110,115, and 120 allow the UE to establish a radio link and are responsiblefor complicated functions as compared to the legacy node Bs. In the LTEsystem, all user traffic including real time services such as Voice overInternet Protocol (VoIP) are provided through a shared channel and thusthere is a need for a device which is located in the eNB to scheduledata based on the state information such as UE buffer conditions, powerheadroom state, and channel state. Typically, one eNB controls aplurality of cells. In order to secure a data rate of up to 100 Mbps,the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM)as a radio access technology. Also, the LTE system adopts AdaptiveModulation and Coding (AMC) to determine the modulation scheme andchannel coding rate in adaptation to the channel condition of the UE.The S-GW 130 is an entity to provide data bearers so as to establish andrelease data bearers under the control of the MME 125. The MME 125 isresponsible for various control functions and connected to a pluralityof eNBs 105, 110, 115, and 120.

FIG. 2 is a diagram illustrating a protocol stack of the LTE systemaccording to the related art.

Referring to FIG. 2, the protocol stack of the LTE system includesPacket Data Convergence Protocol (PDCP) 205 and 240, Radio Link Control(RLC) 210 and 235, Medium Access Control (MAC) 215 and 230, and Physical(PHY) 220 and 225. The PDCP 205 and 240 is responsible for IP headercompression/decompression, and the RLC 210 and 235 is responsible forsegmenting the PDCP Protocol Data Unit (PDU) into segments inappropriate size for Automatic Repeat Request (ARQ) operation. The MAC215 and 230 is responsible for establishing connection to a plurality ofRLC entities so as to multiplex the RLC PDUs into MAC PDUs anddemultiplex the MAC PDUs into RLC PDUs. The PHY 220 and 225 performschannel coding on the MAC PDU and modulates the MAC PDU into OFDMsymbols to transmit over radio channel or performs demodulating andchannel-decoding on the received OFDM symbols and delivers the decodeddata to the higher layer. Also, the PHY layer uses Hybrid ARQ (HARQ) foradditional error correction by transmitting 1 bit information indicatingfor positive or negative acknowledgement from the receiver to thetransmitter. This is referred to as HARQ ACK/NACK information. Thedownlink HARQ ACK/NACK information corresponding to an uplinktransmission is transmitted through Physical Hybrid-ARQ IndicatorChannel (PHICH), and the uplink HARQ ACK/NACK information correspondingto a downlink transmission can be transmitted through Physical UplinkControl Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH).

A UE can be in one of two states: a connected mode and an idle mode.

Meanwhile, an eNB can be categorized into one of the following types: anacceptable cell, a suitable cell, a barred cell, and a reserved cell. Anacceptable cell corresponds to a cell allowing for limited services(emergency call and ETWS), and a cell not barred and satisfying cellselection conditions. A suitable cell corresponds to a cell providing UEwith normal service, a cell which is part of selected/registered PublicLand Mobile Network (PLMN) or PLMN of the equivalent PLMN list, for aCSG cell, CSG ID broadcast by cell is present in CSG white list, and acell not barred and satisfying cell selection conditions. A barred cellcorresponds to a cell indicated as barred cell in system information. Areserved cell corresponds to a cell indicated as reserved cell in systeminformation.

The UE in the idle mode can be in one of several states according to thetype of cell it search for or camps on. For example, when the UE is inthe idle mode, the UE may be in a camped normally state, a cellselection when leaving RRC_Connected state, an any cell selection state,and a camped on any cell state.

The camped normally state corresponds to a state in which a suitablecell is found through the cell selection/reselection procedure. In thisstate, the UE monitors the paging channel to determine whether there isnew data to be received from the network and receives systeminformation. The UE also performs measurement for cell reselection andexecutes a cell reselection procedure.

The cell Selection when leaving RRC_Connected state corresponds to astate in which the UE attempts to search for a suitable cell when ittransitions from RRC_CONNECTED state to the RRC_IDLE state.

The any cell selection state corresponds to a state in which the UEattempts to find an acceptable cell of any PLMN when it fails finding a‘suitable cell’ or is rejected by the selected PLMN, the UE attempts tofine an acceptable cell of any PLMN in this state. When an acceptablecell is found, the UE transitions to the ‘Camped on any cell state.’

The camped on any cell state corresponds to a state in which the UEmonitors paging channel to determine whether there is new data to bereceived from the network and receives system information. The UE alsoperforms measurement for cell reselection and executes cell reselectionprocedure according to conditions. The UE also perform cell search foradjacent suitable cell.

The UE in idle mode receives system information broadcast by the eNB toacquire the information about the neighbor eNBs. The eNB (hereinafter,interchangeably used with the term ‘macro eNB’) notifies the UEs of thePhysical Cell Identifier (PCI) list used by the CSG eNBs through SystemInformation Block (SIB) 4. The PCI list is notified by means of theparameter csg-PhysCellIdRange and, once the csg-PhysCellIdRange isreceived, the UE camped on a cell of the PLMN assumes that the PCI listis value for 24 hours. If the UE is not a member of the CSG cell forthis duration, the cells using the corresponding PCI is ruled out in thecell selection/reselection procedure.

However, the method according to the related art has a drawback in thatthe PCI used by the CSG eNBs is always ruled out in the cell search(selection/reselection) so as to disturb efficient cell search of theUE.

Therefore, a need exists for a cell search method and apparatus of a UEthat is capable of facilitating cell search procedure based on the PCIlist used by the CSG cells in adaptation to the UE status.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present invention.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a cell search method and apparatus of a UserEquipment (UE) that is capable of facilitating cell search procedurebased on the Physical Cell Identifier (PCI) list used by the ClosedSubscriber Group (CSG) cells in adaptation to the UE status.

Another aspect of the present invention is to provide a method for useof the PCI list of CSG cells this is transmitted by an evolved Node B(eNB) as follows.

Unlike the normal situation in which the cell reselection procedure isperformed with the PCI list information used by the CSG cells, if it isin an any cell selection state (e.g., state allowing for only theemergency service), the UE perform the cell reselection procedure to allcells including the CSG cells indicated by the PCI list information(nullification or removal).

The UE determines whether the Primary Public Land Mobile Network (PLMN)is changed and, if so, includes the cells included in the PCI listinformation used by the CSG cells (nullification or removal) in the cellreselection procedure. Conversely, if the Primary PLMN is not changed,the UE performs the cell reselection procedure with the application ofthe PCI list used by the CSG cells.

When the Primary PLMN is changed, if new PCI list information used bythe CSG cells is received the cell, the cell reselection is performedwith the application of the corresponding application.

In accordance with an aspect of the present invention, a cell searchmethod of a terminal in a mobile communication system is provided. Thecell search method includes selecting a cell of a base station,receiving a system information including a Physical Cell Identity (PCI)list used by neighbor Closed Subscribed Group (CSG) cells of the basestation, determining whether the terminal is in any cell selection statefor selecting any cell, and searching, when the terminal is in the anycell selection state, neighbor cells without application of the PCIlist.

In accordance with another aspect of the present invention, a terminalfor searching neighbor cells in a mobile communication system isprovided. The terminal includes a transceiver for transmitting andreceiving signals to and from a base station, and a controller forselecting a cell of a base station, for receiving system informationincluding a Physical Cell Identity (PCI) list used by neighbor ClosedSubscribed Group (CSG) cells of the base station, for determiningwhether the terminal is in an any cell selection state for selecting anycell, and for searching, when the terminal is in the any cell selectionstate, neighbor cells without application of the PCI list.

In accordance with another aspect of the present invention, a neighborcell search method of a terminal in a mobile communication system isprovided. The neighbor cell search method includes selecting a cell of afirst base station, determining, when first system information isreceived, whether a network operator is changed, and searching, when thenetwork operator is changed, neighbor cells without application ofPhysical Cell Identity (PCI) list received from other base station thanthe first base station.

In accordance with still another aspect of the present invention, aterminal for searching neighbor cells in a mobile communication systemis provided. The terminal includes a transceiver for transmitting andreceiving signals to and from a base station, and a controller forselecting a cell of a first base station, for determining, when a firstsystem information is received, whether a network operator is changed,and for searching, when the network operator is changed, neighbor cellswithout application of Physical Cell Identity (PCI) list received fromother base station than the first base station.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a diagram illustrating an architecture of a Long TermEvolution (LTE) system according to the related art;

FIG. 2 is a diagram illustrating a protocol stack of an LTE systemaccording to the related art;

FIG. 3 is a signaling diagram illustrating a Physical Cell Identity(PCI) list-assisted cell search procedure of a User Equipment (UE)according to an exemplary embodiment of the present invention;

FIG. 4 is a flowchart illustrating a PCI list-assisted cell searchprocedure of a UE according to a first exemplary embodiment of thepresent invention;

FIG. 5 is a signaling diagram illustrating a PCI list-assisted cellsearch procedure of a UE according to a second exemplary embodiment ofthe present invention;

FIG. 6 is a flowchart illustrating a PCI list-assisted cell searchprocedure of a UE according to a second exemplary embodiment of thepresent invention;

FIG. 7 is a signaling diagram illustrating a PCI list-assisted cellsearch procedure of a UE according to a third exemplary embodiment ofthe present invention;

FIG. 8 is a flowchart illustrating a PCI list-assisted cell searchprocedure of the procedure of a UE according to the third exemplaryembodiment of the present invention;

FIG. 9 is a block diagram illustrating a configuration of a UE accordingto an exemplary embodiment of the present invention; and

FIG. 10 is a block diagram illustrating a configuration of an evolvedNode B (eNB) according to an exemplary embodiment of the presentinvention.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

As aforementioned, only a Closed Subscriber Group (CSG) member UserEquipments (UEs) are allowed to access a CSG cells and thus there is aneed for a non-CSG member UE to rule out the CSG cells in the cellselection procedure. In the current Long Term Evolution (LTE) standard,a Physical Cell Identity (PCI) list for CSG cells is transmitted from amicro evolved Node B (eNB) to the UE such that the UE searches forneighbor cells with the exception of the CSG cells using the PCI.

However, the PCI list used by the CSG cells can be changed according tothe change of the provider's policy or there can be a special situationrequiring attempt to the CSG cell (e.g. there is no cell for the UE tobe able to access). However, the current technology does not provide anymethod for coping with such a situation and thus there is a need for amethod to overcome this problem.

FIG. 3 is a signaling diagram illustrating a PCI list-assisted cellsearch procedure of the UE according to an exemplary embodiment of thepresent invention. In order to help understand exemplary embodiments ofthe present invention, it is assumed that the UE 301 has completed cellselection/reselection to the eNB 303.

Referring to FIG. 3, the UE 301 in idle mode completes the cellselection/reselection to the eNB 303 at step 311. Thereafter, the UE 301receives the System Information Block (SIB) 4 from the eNB 303 andextracts PCI list used by the neighbor CSG cell from the SIB 4 at step313. The PCI list used by the neighbor CSG cells is included in thecsg-PhysCellIdRange field.

If the csg-PhysCellIdRange field is received, then the UE 301 starts atimer set to a first time duration (e.g., 24 hours) at step 315. Whilethe timer operates, the UE 301 regards that the PCI list used by theneighbor CSG cells which has been received from the eNB 303 is valid. Inthe case in which it measures the neighbor cells for neighbor cellselection/reselection, the UE performs measurement to the cells that arenot included in the PIC list used by the neighbor CSG cells at step 321.

This operation causes no problem in the normal situations, (i.e., whenthere is a cell from which it is possible to receive the normalservice). For example, in a case in which there is a suitable cell amongthe cells with the exception of the cells included in thecsg-PhysCellIdRange, the UE 301 is capable of camping on thecorresponding cell to receive the normal service.

However, if there is no suitable cell among the cells with the exceptionof the cells included in the csg-PhysCellIdRange, then the cell includedin the previously received csg-PhysCellIdRange may be a macro cellrather than a CSG cell and it is preferred that even the emergency callis served by the cell having the best radio channel condition (althoughit is a CSG cell).

Accordingly, when the UE 301 is in an Any Cell Selection state(hereinafter, interchangeably used with the term ‘first state’) as theabove-described state (e.g., when there is no adjacent suitable cell),the UE determines that the csg-PhysCellIdRange information included inthe PCI list is invalid and thus measures neighbor cells including thecells in the csg-PhysCellIdRange to perform cell selection/reselectionat step 323. For example, the UE 301 measures the neighbor cells thatare not indicated in the PCI list in the above case.

If the timer 315 has expired at step 317, then the UE 301 determinesthat the previously received csg-PhysCellIdRange information is notvalid any longer and measures the all neighbor cells including the CSGcells indicated in csg-PhysCellIdRange to perform cellselection/reselection at step 325.

FIG. 4 is a flowchart illustrating a PCI list-assisted cell searchprocedure of a UE according to a first exemplary embodiment of thepresent invention.

Referring to FIG. 4, the UE 301 receives a Physical Cell ID rangeinformation for used in CSG cells (i.e., PCI list in the SIB4 from theeNB 303 (e.g., macro eNB)) at step 401. Upon receipt of the PCI list,the UE 301 starts the timer having a length of the first time duration(e.g., 24 hours) at step 403.

At step 405, the UE 301 determines whether the timer has expired. If thetimer is determined to have expired, then the UE 301 proceeds to step407. In contrast, if the timer is determined to have not expired, thenthe UE 301 proceeds to step 409.

At step 409, the UE 301 determines whether the UE 301 is in the Any CellSelection state.

Afterward, if the timer is running in the cell selection/reselectionprocedure and if the UE is not in the Any Cell Selection state in whichno adjacent suitable cell exists, then the UE 301 regards that theinformation received at step 401 as valid and uses this information inthe neighbor cell selection/reselection procedure at step 411. Forexample, if it does not belongs to any CSG cell, then the UE 301performs cell selection/reselection procedure with the exception of theCSG cells indicated in the PCI list.

However, if the time has expired or if the UE is in the Any CellSelection state having no suitable cells around although the timer hasnot expired, then the UE 301 regards that the information received atstep 401 as invalid and thus rules out the Physical Cell id range (i.e.,the PCI list) in the cell selection/reselection procedure. In otherwords, the UE 301 measures neighbor cells without consideration of thePCI list.

FIG. 5 is a signaling diagram illustrating a PCI list-assisted cellsearch procedure of a UE according to a second exemplary embodiment ofthe present invention. In order to help understand an exemplaryembodiment of the present invention, it is assumed that the UE is notthe member of any CSG.

Referring to FIG. 5, the UE 501 in idle mode completes the cellselection/reselection to the eNB A 503 at step 511. Once the cellselection/reselection procedure has completed, the UE 501 receives anSIB 1 message from the eNB A 503 to acquire Public Land Mobile Network(PLMN) information of the eNB A 503 at step 513. The UE 501 compares thereceived PLMN information with the Primary PLMN information of the eNBto determine whether the PLMN is changed at step 515.

Upon receipt of the SIB 4 which includes csg-PhysCellIdRange informationat step 517, if the PLMN is not changed, then the UE 501 starts a timerset to a first duration (e.g., 24 hours)at step 519. The SIB 4information can be received from the other eNB to which the UE 501 hasattached than eNB A 503 and, if the Primary PLMN has not been changed,then the timer started or restarted at the receipt of the SIB 4 ismaintained with the validity of the csg-PhysCellIdRange information atstep 521.

Afterward, the UE 501 performs cell reselection to the eNB B 505according to the roaming or change of the radio channel condition atstep 531. The UE 501 receives SIB 1 message from the newly selected eNBB 505 to acquire the PLMN information of eNB B 505 at step 533.

Next, the UE 501 compares the received PLMN information with the PrimaryPLMN of the old eNB to determine whether the PLMN is changed at step535. If the PLMN is changed, then the UE 501 measures the neighbor cellat step 537 without consideration of the csg-PhysCellIdRange received atstep 517. For example, the UE 501 performs neighbor cell measurementwithout application of the PCI list. However, if the csg-PhysCellIdRangeis acquired from the SIB 4 received from the eNB B 505 at step 539, thenthe UE 501 performs measurement for cell selection/reselection inconsideration of the newly received csg-PhysCellIdRange at step 543. Forexample, the UE 501 rules out the CSG cells indicated in thecsg-PhysCellIdRange (or PCI list) in the measurement. Thecsg-PhysCellIdRange received at step 517 and the csg-PhysCellIdRangereceived at step 539 can be identical with or different from each other.Upon receipt of the SIB 4 which includes csg-PhysCellIdRange informationat step 539, the UE 501 starts a timer set to a first duration (e.g., 24hours) at step 541.

FIG. 6 is a flowchart illustrating a PCI list-assisted cell searchprocedure of a UE according to a second exemplary embodiment of thepresent invention.

Referring to FIG. 6, the UE completes cell selection/reselection to anew cell at step 601. Once the cell selection/reselection to a new cellhas completed at step 601, the UE 501 receives PLMN information in theSIB1 transmitted by the newly selected/reselected cell (e.g., eNB).Next, the UE 501 determines whether the PLMNs of the old cell (e.g.,previously selected/reselected) and the new cell match with each otherat step 603.

If the PLMNs are determined to not match with each other, the UE 501determines whether the Physical Cell id range information receivedpreviously from other CSG cell exists at step 605. If the previouslystore Physical Cell id range information is determined to exist, the UEdiscards this information at step 607. In contrast, if the Physical Cellid range information received previously from other CSG cell isdetermined to not exist at step 605, then the procedure jumps to step609. Similarly, if the PLMNs match with each other at step 603, theprocedure jumps to step 609.

At step 609, the UE 501 determines whether new Physical Cell id rangeinformation on the CSG cell is received in the SIB4 transmitted by thenewly selected/reselected cell. If new Physical Cell id rangeinformation is determined to have been received at step 609, the UE 501uses the corresponding information in the cell selection/reselectionprocedure at step 611.

In contrast, if no new Physical Cell id range information is determinedto have been received at step 609, the UE 501 determines whether thePhysical Cell id range information of the old CSG cell is received atstep 613. If the Physical Cell id range information of the old CSG cellis determined to have been received at step 613, the UE 501 performs thecell selection/reselection procedure with the old Physical Cell id rangeinformation at step 615. In contrast, if the Physical Cell id rangeinformation of the old CSG is determined to have not been received atstep 613, then the UE 501 performs cell selection/reselection procedureto all cells at step 617.

FIG. 7 is a signaling diagram illustrating a PCI list-assisted cellsearch procedure of a UE according to a third exemplary embodiment ofthe present invention. In order to help understand the third exemplaryembodiment of the present invention, it is assumed that the UE is notthe member of any CSG.

Referring FIG. 7, the UE 701 in idle mode completes the cellselection/reselection to the eNB 703 at step 711.

After completing the cell selection/reselection, the UE 701 receives anSIB 1 message from the eNB 703 to acquire the PLMN information at step713.

The UE 701 compares the received PLMN information and the Primary PLMNinformation of the old eNB to determine whether the PLMN is changed atstep 715.

If the PLMN is changed, the UE 701 measures the neighbor cells withoutconsideration of the csg-PhysCellIdRange information received from theold cell at step 717. For example, the UE 701 performs neighbor cellmeasurement without application of the PCI list. However, if thecsg-PhysCellIdRange is received in the SIB4 from the eNB 703 afterwardat step 719, the UE 701 performs measurement for cellselection/reselection in consideration of the newly receivedcsg-PhysCellIdRange at step 723. For example, the UE 701 rules out theCSG cells indicated by the csg-PhysCellIdRange in the measurement.

If the csg-PhysCellIdRange is received, the UE 701 starts a timer havinga length of the first time duration (e.g., 24 hours) at step 721. Untilthe timer expires at step 727, the PCI list used by the neighbor CSGcells is valid. Accordingly, when measuring the neighbor cells for cellselection/reselection, the UE 701 performs measurement to the cells thatare not included in the PCI list at step 723.

This operation does not cause a problem when there is a cell providingthe normal service. For example, if there is a suitable cell among thecells that are not indicated by the csg-PhysCellIdRange, the UE 701camps on the corresponding cells to receive the normal service.

However, the cell indicated by the csg-PhysCellIdRange which has beenreceived previously in the situation in which there is no suitable cellamong the cells that are not indicated by the csg-PhysCellIdRange may bea normal macro cell and it is preferred that even the emergency call isserved by the cell having the best radio channel condition (although itis a CSG cell).

In addition, at step 725, the UE 701 performs a measurement for cellselection/reselection when the UE is in an Any Cell Selection state.

Accordingly, when the UE is in the Any Cell Selection state as abovedescribed (e.g., when there is no suitable cell adjacent), the UE 701determines the invalidity of the csg-PhysCellIdRange and performsmeasurement to the neighbor cells including the cells indicated by thecsg-PhysCellIdRange for cell selection/reselection at step 729.

FIG. 8 is a flowchart illustrating a PCI list-assisted cell searchprocedure of the procedure of a UE according to a third exemplaryembodiment of the present invention.

Referring to FIG. 8, the UE completes cell selection/reselection at step801. After selecting/reselecting a new cell at step 801, the UE receivesPLMN information in the SIB1 transmitted by the newlyselected/reselected cell and determines whether the received PLMNinformation is identical with the Primary PLMN of the old cell at step803.

If the PLMNs are determined to not match each other, the UE determineswhether the Physical Cell id range information of CSG cells which hasbeen received previously from other cell exists at step 805. If thePhysical Cell id range information is determined to exist, the UEdiscards this Physical Cell id range information at step 807.

In contrast, if the UE determines that the Physical Cell id rangeinformation of CSG cells which has been received previously from othercell does not exist at step 805, then the procedure jumps to step 809.Similarly, if the PLMNs are determined to match each other at step 803,the procedure jumps to step 809.

The UE determines whether the Physical Cell id range information used inthe CSG cells is received newly in the SIB4 from the newlyselected/reselected cell at step 809. If the UE determines that thePhysical Cell id range information used in the CSG cells is receivednewly in the SIB4 from the newly selected/reselected cell at step 809,then the UE starts a timer having the length of the first time duration(e.g., 24 hours) at step 811.

The UE determines whether the timer has expired at step 813. If thetimer is determined to have not expired, the UE determines whether theUE is in a state capable of receiving emergency call service (e.g., theAny cell selection state) at step 815. If the UE is determined to not bein the Any Cell Selection state in which no adjacent suitable cellexists, the UE regards that the physical Cell id range information usedin the CSG cells is valid at step 817. The UE then performs cellselection/reselection by referencing the newly received Physical Cell idrange information afterward at step 819. For example, the CSG cellsincluded in the received PCI list are ruled out in measurement.

However, if the timer is determined to have expired at step 813, if thetimer has not expired but the UE is determined to be in the Any CellSelection state in which no adjacent suitable cell exists at step 815,the UE regards at step 821 that the information received at step 809 isinvalid. Thus the UE performs cell selection/reselection to all cellswithout consideration of the Physical Cell id range at step 823.

If the UE determines that no new Physical Cell id range information ofCSG cells is received in SIB4 at step 809, the UE determines whether theold Physical Cell id range information of the CSG cells is received atstep 825. If the old Physical Cell id range information is determined tohave been received at step 825, the UE performs cellselection/reselection procedure with the old Physical Cell id rangeinformation at step 827. In contrast, if the old Physical Cell id rangeinformation is determined to have not been received at step 825,performs the cell selection/reselection procedure to all cells at step823.

FIG. 9 is a block diagram illustrating a configuration of a UE accordingto an exemplary embodiment of the present invention.

Referring to FIG. 9, the UE according to an exemplary embodiment of thepresent invention includes a transceiver 905, a controller 910, amultiplexer/demultiplexer 920, higher layer processors 925 and 930, anda control message processor 935.

The transceiver 905 receives data and control signals through a downlinkchannel of the serving cell and transmits data and control signalthrough an uplink channel. If multiple serving cells are configured, thetransceiver 905 is capable of transmitting and receiving data andcontrol signals through multiple serving cells.

The multiplexer/demultiplexer 920 multiplexes the data generated by thehigher layer processor 925 and 930 and the control message processor935, and demultiplexes the data received by the transceiver 905 todeliver the demultiplexed data to appropriate processors (e.g., thehigher layer processors 925 and 930 and the control message processor935).

The control message processor 935 processes the control message receivedfrom the eNB to take an appropriate action. For example, if DRX-relatedparameters are received, the control message processor 935 deliversthese parameters to the controller 910.

The higher layer processors 925 or 930 are configured per service toprocess the data generated by a user service such as File TransferProtocol (FTP) and Voice over Internet Protocol (VoIP) and to transferthe processed data to the multiplexer/demultiplexer 920 and to processthe data from the multiplexer/demultiplexer 920 and to deliver theprocessed data to the service application running on the higher layer.

The controller 910 analyzes the scheduling command (e.g., uplink grant),received by the transceiver 905, and controls the transceiver 905 andthe multiplexer/demultiplexer 920 to perform uplink transmission withappropriate transmission resource at appropriate timing. The controller910 controls the transceiver 905 by taking notice of the DRX operationand CSI/SRS transmission.

According to an exemplary embodiment of the present invention, thecontroller 910 is capable of performing cell selection to a certain eNB.The controller controls receiving the system information includingPhysical Cell Identity (PCI) used by adjacent Closed Subscriber Group(CSG) from the eNB. The controller 910 determines whether the UE is inan Any Cell Selection state and, if so, controls the neighbor cellsearch without application of the received PCI list. In this case, ifthe UE performs neighbor cell search without application of the receivedPCI list, then the neighbor cell search is performed to all cellsincluding the CSG cells included in the PCI list.

If the UE is not in the Any Cell Selection state, the controller 910controls the neighbor cell search with application of the received PCIlist. In this case, if the UE performs neighbor cell search withapplication of the received PCI list, this means that the neighbor cellsearch is performed to the cells with the exception of CSG cellsincluded in the PCI list.

The controller 910 is also capable of starting a timer having the lengthof the first time duration after receipt of the system information andcontrolling the neighbor cell search with application of the PCI listbefore the timer expires.

According to another exemplary embodiment of the present invention, thecontroller 910 is capable of performing cell search to a certain firsteNB. If the first system information is received from the first eNB, thecontroller 910 determines whether the PLMN is changed and, if so,controls the neighbor cell search without applying Physical CellIdentity (PCI) list received from the first eNB and other eNBs.

If the second system information including the PCI list used by theClosed Subscriber Group (CSG) cells which is transmitted by the firsteNB is received, the controller 910 is capable of performing neighborcell search with application of the PCI list included in the secondsystem information.

FIG. 10 is a block diagram illustrating a configuration of an eNBaccording to an exemplary embodiment of the present invention.

Referring to FIG. 10, the eNB includes a transceiver 1005, a controller1010, a scheduler 1015, a multiplexer/demultiplexer 1020, various higherlayer processors 1025 and 1030, and a control message processor 1035.

The transceiver 1005 transmits data and control signals on a downlinkcarrier and receives data and control signals on an uplink carrier. Ifmultiple carriers are configured, the transceiver 1005 transmits andreceives the data and control signals on the multiple carriers.

The multiplexer/demultiplexer 1020 multiplexes the data generated by thehigher layer processors 1025 and 1030 and the control message processor1035, and demultiplexes the data received by the transceiver 1005 todeliver the demultiplexed data to at least one of the higher layerprocessors 1025 and 1030, the control message processor 1035, and thecontroller 1010. The control message processor 1035 processes themessage transmitted by the UE and takes a necessary action or generatesa control message to be transmitted to the UE to the higher layer.

The higher layer processors 1025 or 1030 are configured per UE perservice to process the data generated by a user service such as FileTransfer Protocol (FTP) and Voice over Internet Protocol (VoIP), totransfer the processed data to the multiplexer/demultiplexer 1020, toprocess the data from the multiplexer/demultiplexer 1020, and to deliverthe processed data to the service application running on the higherlayer.

The controller 1010 controls the transceiver by taking notice of theCSI/SRS transmission timing of the UE.

The scheduler 1015 allocates transmission resource to the UE at anappropriate time in consideration of the buffer status of the UE,channel status, and active time of the UE; and controls the transceiverto process the signals transmitted by the UE or to be transmitted to theUE.

As described above, the cell search method and apparatus of exemplaryembodiments of the present invention is capable of performing the cellsearch procedure to appropriate eNBs, resulting in improvement ofnetwork attachment efficiency.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A cell search method of a terminal in a mobilecommunication system, the cell search method comprising: selecting acell of a base station; receiving system information including aPhysical Cell Identity (PCI) list used by neighbor Closed SubscribedGroup (CSG) cells of the base station; determining whether the terminalis in an any cell selection state for selecting any cell; and searching,when the terminal is in the any cell selection state, neighbor cellswithout application of the PCI list.
 2. The cell search method of claim1, further comprising: searching, when the terminal is not in the anycell selection state, the neighbor cells with application of the PCIlist.
 3. The cell search method of claim 2, wherein the searching of theneighbor cells with application of the PCI list comprises searching theneighbor cells with the exception of the CSG cells belonging to the PCIlist.
 4. The cell search method of claim 1, wherein the searching of theneighbor cells without application of the PCI list comprises searchingthe neighbor cells including the CSG cells belonging to the PCI list. 5.The cell search method of claim 1, further comprising: starting, uponreceiving the system information, a timer having a length of a firsttime duration; and searching the neighbor cells with application of thePCI list before the timer expires.
 6. The cell search method of claim 1,wherein the PCI list is included in a csg-PhysCellIdRange parameter ofthe system information.
 7. A terminal for searching neighbor cells in amobile communication system, the terminal comprising: a transceiver fortransmitting and receiving signals to and from a base station; and acontroller for selecting a cell of a base station, for receiving asystem information including a Physical Cell Identity (PCI) list used byneighbor Closed Subscribed Group (CSG) cells of the base station, fordetermining whether the terminal is in an any cell selection state forselecting any cell, and for searching, when the terminal is in the anycell selection state, neighbor cells without application of the PCIlist.
 8. The terminal of claim 7, wherein the controller is configuredfor searching, when the terminal is not in the any cell selection state,the neighbor cells with application of the PCI list.
 9. The terminal ofclaim 8, wherein the neighbor cells, when the PCI list is applied,comprises cells with the exception of the CSG cells belonging to the PCIlist.
 10. The terminal of claim 7, wherein the neighbor cells, when thePCI list is not applied, comprises the CSG cells belonging to the PCIlist.
 11. The terminal of claim 7, wherein the controller is configuredfor starting, upon receiving the system information, a timer having alength of a first time duration, and for searching the neighbor cellswith application of the PCI list before the timer expires.
 12. Theterminal of claim 7, wherein the PCI list is included in acsg-PhysCellIdRange parameter of the system information.
 13. A neighborcell search method of a terminal in a mobile communication system, theneighbor cell search method comprising: selecting a cell of a first basestation; determining, when first system information is received, whethera network operator is changed; and searching, when the network operatoris changed, neighbor cells without application of Physical Cell Identity(PCI) list received from other base station than the first base station.14. The neighbor cell search method of claim 13, further comprisingsearching, when second system information including the PCI list used byneighbor Closed Subscriber Group (CSG) cells of the first base stationis received, neighbor cells with application of the PCI list included inthe second system information.
 15. The neighbor cell search method ofclaim 14, wherein the searching of neighbor cell with application of thePCI list comprises searching the cells with the exception of the CSGcells belonging to the PCI list.
 16. The neighbor cell search method ofclaim 13, wherein the searching of neighbor cells without application ofPhysical Cell Identity (PCI) list received from other base station thanthe first base station comprises searching the neighbor cells includingClosed Subscriber Group (CSG) cells belonging to the PCI list.
 17. Aterminal for searching neighbor cells in a mobile communication system,the terminal comprising: a transceiver for transmitting and receivingsignals to and from a base station; and a controller for selecting acell of a first base station, for determining, when first systeminformation is received, whether a network operator is changed, and forsearching, when the network operator is changed, neighbor cells withoutapplication of Physical Cell Identity (PCI) list received from otherbase station than the first base station.
 18. The terminal of claim 17,wherein the controller is configured for searching, when second systeminformation including the PCI list used by neighbor Closed SubscriberGroup (CSG) cells of the first base station is received, neighbor cellswith application of the PCI list included in the second systeminformation.
 19. The terminal of claim 17, wherein the neighbor cells,when the PCI list is not applied, comprises the CSG cells belonging tothe PCI list.
 20. The terminal of claim 18, wherein the neighbor cells,when the PCI list is applied, comprises cells with the exception of theCSG cells belonging to the PCI list.